The synchronous speed of A.C. motors is proportional to the frequency of the applied power. The use of an adjustable frequency power supply to control the speed of A.C. motors is well-known. Such power supplies typically include a rectifier to convert the normal 60 Hz. alternating current (A.C.) power to direct current (D.C.). an inverter to convert the D.C. to A.C. at the desired frequency, and control circuitry to provide the signals necessary to synthesize the correct frequency to achieve a given desired motor speed.
In order to assure that a constant flux density is maintained throughout the range over which the frequency is to vary to achieve the desired speed control, the voltage applied to the motor should bear a relationship to the frequency. Typical motor speed control configurations are designed to maintain a constant value of volts per hertz; i.e., the quantity V.sub.t /f where V.sub.t is the terminal voltage applied to the motor and f is the frequency of the applied A.C. power.
In order to maintain this constant V.sub.t /f, A.C. motor speed control systems often employ a chopper circuit in between the rectifier and the inverter to deliver variable voltage D.C. power to the inverter to compensate for changes in frequency.
Typical motor speed control inverters have used thyristors and transistors as switching elements. In order to supply signals to turn on these devices it has been necessary to use isolation devices, such as optical couplers or transformers, and one or more separate power supplies for the gate or base drive circuits of the switching elements. These schemes have proved to be adequate for the task, but there has remained a need to accomplish inverter switching with a minimum number of components to minimize the system cost, while at the same time maintaining reliability and minimum drive requirements.
Some variable frequency A.C. motor speed control schemes are implemented for energy savings. Industrial applications of A.C. motors represent a major use of electrical energy. As the cost of energy increases, the industry, recognizing that there is a potential for energy savings in motor applications where motor loads vary, such as fan or pump loads, has begun to realize energy saving by varying the speed of motors where possible. This energy saving results directly in cost savings.
There are, however, situations where a motor is capable of supplying more torque than is required by the load at a given speed. This additional torque represents wasted energy since it is not necessary to drive the load. There thus exists a need for a means to tailor the motor torque to the load requirements in order to realize the maximum energy savings possible.
It is an object of this invention to provide an improved base drive circuit for the drive transistors in an A.C. motor speed control inverter.
It is a further object of this invention to provide a reliable base drive circuit for A.C. motor speed control inverters which utilizes a minimum number of low-cost components.
Another object of this invention is to provide an A.C. motor speed control system which dynamically tailors the torque delivered by the motor to the load requirements.
Yet another object of the invention is to provide an A.C. motor speed control system which minimizes unnecessary use of electrical energy to drive the motor.